A fast neutron reactor is a nuclear reactor in which the fission chain reaction is sustained by fast neutrons. That means the neutron moderator (slowing down) in such reactors is undesirable. This is a key advantage of fast reactors, because fast reactors have a significant excess of neutrons (due to low parasitic absorbtion), unlike PWRs (or LWRs).

Sodium-cooled Fast Reactor (SFR).Source: wikipedia.org

On the other hand such reactors must compensate for the missing reactivity from neutron moderator efect. They use fuel with higher enrichment when compared to that required for a thermal reactor. Fast reactors require enrichments about 10%, or more. Most fast reactors use a hexagonal lattice cells (as VVER reactors) in order to reach smaller volume ratios of coolant to fuel. Generally, fast reactors have to utilize much more compact nuclear cores than thermal reactors (PWRs or BWRs) in order to reach required core reactivity. This implies the fast reactor cores achieve higher power densities. As a consequence, they cannot use water as coolant, because of its moderating properties and insufficient thermal properties. The solution given this problem is to use another coolant as liquid sodium or lead.

Lead-cooled Fast Reactor (LFR).Source: wikipedia.org

Fast reactorfuel may be metal or a ceramic, encapsulated in metal cladding, unlike the PWR’s zirconium cladding. Liquid metals are the most widely used coolant because they have excellent heat transfer properties and can be employed in lowpressure systems. Sodium-cooled fast reactors (SFRs) are the most common designs. Because sodium reacts violently with water, however, SFRs require the placement of an intermediate heat exchanger between the reactor core and the steam generator. This hi-tech technology requires a lot of experience, therefore only few countries have developed their own fast reactor design (e.g. Russia, USA, France, Japan, ). Especially Russians continue in fast reactor developement program with their BN reactors.

Breeder reactor

A breeder reactor is essentially a particular configuration of a fast reactor (but not only FBR can be used as a breeder). Fast reactors generally have an excess of neutrons (due to low parasitic absorbtion), the neutrons given off by fission reactions can “breed” more fuel from otherwise non-fissionable isotopes or can be used for another purposes (e.g.transmutation of spent nuclear fuel). The most common breeding reaction is an absorbtion reaction on uranium-238, where a plutonium-239 from non-fissionable uranium-238 is produced. A key parameter of breeder reactors is a breeding ratio, although this ratio describes also thermal reactors fuel cycle.

The term “breeder” refers to the types of configurations which can be the breeding ratio higher than 1. That means such reactors produce more fissionable fuel than they consume (i.e. more fissionable Pu-239 is produced from non-fissionable uranium-238, than consumed initial U-235+Pu-239 fuel).

Disadvantages

Generation IV reactors

In 2003 the Generation IV International Forum (GIF) representing ten countries announced the selection of six reactor technologies which they believe represent the future shape of nuclear energy. These were selected on the basis of being clean, safe and cost-effective means of meeting increased energy. Three of the six reactors are fast reactors and one can be built as a fast reactor, one is described as epithermal. Only two operate with slow neutrons like today’s plants.

1. Question

1 points

These nuclear reactions are fast and involve a single-nucleon interaction. The interaction time must be very short (~10-22 s). Incident particles interact on the surface of a target nucleus rather than in the volume of a target nucleus.

3. Question

1 points

Products of these reactions are distributed near isotropically in angle (the nucleus loses memory of how it was created – the Bohr’s hypothesis of independence). The mode of decay of the nucleus do not depend on the way the nucleus is formed.

The neutron elastic scattering can occur by way of two interaction mechanisms:

Potential scattering. In potential scattering, the neutron and the nucleus interact without neutron absorption and the formation of a compound nucleus. In fact, the incident neutron does not necessarily have to “touch” the nucleus and the neutron is scattered by the short range nuclear forces when it approaches close enough to the nucleus. Potential scattering occurs with incident neutrons that have an energy of up to about 1 MeV. It may be modeled as a billiard ball collision between a neutron and a nucleus.

Compound-elastic scattering. In some cases, if the kinetic energy of an incident neutron just right to form a resonance, the neutron may be absorbed and a compound nucleus may be formed. This interaction is more unusual and is also known as resonance elastic scattering. Due to formation of the compound nucleus, initial and final neutron are not the same.

The neutron elastic scattering can occur by way of two interaction mechanisms:

Potential scattering. In potential scattering, the neutron and the nucleus interact without neutron absorption and the formation of a compound nucleus. In fact, the incident neutron does not necessarily have to “touch” the nucleus and the neutron is scattered by the short range nuclear forces when it approaches close enough to the nucleus. Potential scattering occurs with incident neutrons that have an energy of up to about 1 MeV. It may be modeled as a billiard ball collision between a neutron and a nucleus.

Compound-elastic scattering. In some cases, if the kinetic energy of an incident neutron just right to form a resonance, the neutron may be absorbed and a compound nucleus may be formed. This interaction is more unusual and is also known as resonance elastic scattering. Due to formation of the compound nucleus, initial and final neutron are not the same.

Question 10 of 20

10. Question

1 points

Elastic scattering (cross-section) of slow neutrons by molecules is greater than by free nuclei.

If the kinetic energy of an incident neutron is large compared with the chemical binding energy of the atoms in a molecule, the chemical bound can be ignored.If the kinetic energy of an incident neutron is of the order or less than the chemical binding energy, the cross-section of the molecule is not equal to the sum of cross-sections of its individual nuclei.

Scattering of slow neutrons by molecules is greater than by free nuclei.

If the kinetic energy of an incident neutron is large compared with the chemical binding energy of the atoms in a molecule, the chemical bound can be ignored.If the kinetic energy of an incident neutron is of the order or less than the chemical binding energy, the cross-section of the molecule is not equal to the sum of cross-sections of its individual nuclei.

Scattering of slow neutrons by molecules is greater than by free nuclei.

Question 11 of 20

11. Question

1 points

During an inelastic scattering the neutron can be absorbed and then re-emitted. This reaction belongs to direct reactions.

15. Question

There is a cadmium cut-off energy (Cadmium edge) in the absorption cross-section. Only neutrons of kinetic energy below the cadmium cut-off energy (~0.5 eV) are strongly absorbed by 113Cd. Therefore cadmium is widely used to absorb thermal neutrons in a thermal neutron filters.

There is a cadmium cut-off energy (Cadmium edge) in the absorption cross-section. Only neutrons of kinetic energy below the cadmium cut-off energy (~0.5 eV) are strongly absorbed by 113Cd. Therefore cadmium is widely used to absorb thermal neutrons in a thermal neutron filters.

Question 16 of 20

16. Question

1 points

What is not typical for (γ, n) reactions (production of photoneutrons)?

(γ, n) reactions are threshold reactions.

Photoneutrons do not contribute to neutron flux in a reactor when the reactor is in long term shutdown.

These reactions play a significant role also in reactor kinetics and in a subcriticality control.

The lowest threshold energy for (γ, n) reaction have 9Be with 1.666 MeV and 2D with 2.226 MeV.

18. Question

Explanation: Most of (n,alpha) reactions of thermal neutrons are 10B(n,alpha)7Li reactions accompanied by 0.48 MeV gamma emission. Therefore boron can be used as neutron absorber in case chemical shim, burnable absorbers or control rods. It can be also used as the neutron converter in neutron detectors, because its cross-section is very high and produces energetic alpha particles.

Explanation: Most of (n,alpha) reactions of thermal neutrons are 10B(n,alpha)7Li reactions accompanied by 0.48 MeV gamma emission. Therefore boron can be used as neutron absorber in case chemical shim, burnable absorbers or control rods. It can be also used as the neutron converter in neutron detectors, because its cross-section is very high and produces energetic alpha particles.

Question 19 of 20

19. Question

1 points

Which of the following charged particle reactions is very important in radiation protection?

Explanation: This nuclear reaction continually take place especially in the earth’s atmosphere, forming equilibrium amounts of the radionuclide 14C. In nuclear power plants, it is important especially from radiation protection point of view. The reaction is responsible for most of the radiation dose delivered to the human body by thermal neutrons. The nitrogen atoms are contained in proteins, therefore if the human body is exposed to thermal neutrons, then these thermal neutrons may be absorbed by 14N, causing a proton emission. Protons are directly ionizing particles and deposit their energy over a very short distance in the body tissue.

Explanation: This nuclear reaction continually take place especially in the earth’s atmosphere, forming equilibrium amounts of the radionuclide 14C. In nuclear power plants, it is important especially from radiation protection point of view. The reaction is responsible for most of the radiation dose delivered to the human body by thermal neutrons. The nitrogen atoms are contained in proteins, therefore if the human body is exposed to thermal neutrons, then these thermal neutrons may be absorbed by 14N, causing a proton emission. Protons are directly ionizing particles and deposit their energy over a very short distance in the body tissue.

Question 20 of 20

20. Question

1 points

Q-value of the reaction is:

the difference between the sum of the masses of the initial reactants and the sum of the masses of the final products, in energy units (usually in MeV).

the difference between the sum of the electric charges of the initial reactants and the sum of the electric charges of the final products.

the difference between the sum of the baryon numbers of the initial reactants and the sum of the baryon numbers of the final products.

For reactions in which there is an increase in the kinetic energy of the products Q is positive. The positive Q reactions are said to be exothermic (or exergic). There is a net release of energy, since the kinetic energy of the final state is greater than the kinetic energy of the initial state.

For reactions in which there is a decrease in the kinetic energy of the products Q is negative. The negative Q reactions are said to be endothermic (or endoergic) and they require a net energy input.

For reactions in which there is an increase in the kinetic energy of the products Q is positive. The positive Q reactions are said to be exothermic (or exergic). There is a net release of energy, since the kinetic energy of the final state is greater than the kinetic energy of the initial state.

For reactions in which there is a decrease in the kinetic energy of the products Q is negative. The negative Q reactions are said to be endothermic (or endoergic) and they require a net energy input.

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